1. Field of the Invention
This invention relates to video signal transmission. Particular application is found for this invention in video conference arrangements which connect groups of conferees in a manner that approaches true face-to-face conference situations.
2. Description of the Prior Art
Typical video conference situations involve two or more locations and more than one person in at least one of the locations. To date, such video conference systems have been employing more than one monitor to display the signals of the various locations. Sometimes more than one camera is used within a location when many people are present (with a corresponding increase in the number of monitors). A drawback of such arrangements is the large cumulative bandwidth required for transmitting the various video signals. Consequently, a number of techniques have been developed to reduce the necessary bandwidth.
One such technique, known as temporal resolution, simply transmits the signal of only one camera at a time. Conventionally, the conferee who is speaking loudest is electronically selected and his picture is transmitted to the other conferees. As different people in the group speak, the appropriate camera is selectively enabled and the new speakers' signal is transmitted. One such system is described by R. C. Edson et al., in U.S. Pat. No. 3,601,530, issued Aug. 24, 1971. The drawback of this system is that it transmits only one person at at time, neglecting all other conferees; providing less than optimum video presence. The conferees do not get the feeling that they are present at a conference, because only one of them is visible at any one time. The movement, speech, and other nonverbal information of other conferees are lost.
Another technique, called spatial reduction, reduces the information content of the video signal of each camera by discarding a portion of the picture being sent. One such system is described in a publication by A. J. Seyler et al., "the APO TV-Conferencing Facility," Telecommunications Journal of Australia, Volume 23, No. 3, 1973, pp. 216-225. That system attains a bandwidth reduction by accepting one half of the video picture of two cameras and combining the two half pictures into one video signal. While that system improves video presence and reduces bandwidth, unacceptable picture quality may result as the number of TV cameras, or the number of people per camera increases.
Yet another technique for reducing bandwidth, which may be called time stretching, also reduces the information content of the video signals. One embodiment of this technique is described by van Buul et al., in Philips Research Reports, Volume 28, August 1973, pp. 377-390. In accordance with this technique, only odd-numbered scanning lines are transmitted and, within the monitor, the odd-numbered lines are placed in a pair of analog shift registers. The shift registers are read out seriatim, providing thereby the required number of scan lines. This technique achieves bandwidth reduction but results in a certain degree of picture degradation.
Still another technique for reducing bandwidth, called conditional replenishment, transmits for each frame only the signals that correspond to changes in the picture from the previous frame. This, of course, requires the monitors to have memory to refresh the screen and to replenish the received picture changes. One system employing this technique is described, for example, in U.S. Pat. No. 3,609,244, titled "Conditional Replenishment Video System with Variable Length Address Code", issued to F. W. Mounts on Sept. 28, 1971.
The above techniques are not mutually exclusive and, indeed, some have been combined in the prior art. For instance, in my U.S. Pat. No. 4,004,084, issued Jan. 18, 1979, my colleagues and I have disclosed a system employing temporal and spatial resolution reduction. Therein, a picture from one or more of the cameras is automatically adapted to a speech level control signal or to a motion detector signal so as to advantageously interleave and give priority to a picture from one camera over the pictures from other cameras. The spatial reduction of each picture is realized by transmitting and displaying less than the total picture.
Recently, a new video camera was introduced which develops 500 scan lines for each frame and resolves 1500 pels in each line. This camera has about six times the horizontal resolution of conventional cameras but, correspondingly, it requires about six times the bandwidth of conventional cameras. The prior art bandwidth reduction techniques are inappropriate for this camera because they depend either on the comparison of the signals of separate cameras and/or they degrade the entire video signal of non-selected cameras.
It is an objective of this invention to realize a teleconferencing arrangement with improved video presence and with reduced bandwidth which could advantageously be used with conventional video cameras and with the high resolution video cameras.